Attention has been recently focused on a secondary electron emitter which is used for an electron tube and uses diamond. The reason for this is that the diamond has negative electron affinity, and the diamond has high secondary electron-emission efficiency. One example is reported in “Thin Solid Films 253(1994) p 151.” In the example, the diamond is used as a material for a reflection type secondary electron emitter of which the surface of emission for emitting secondary electrons is the same as the surface of incidence for making primary electrons incident thereon. That is, in the secondary electron emitter, a polycrystalline diamond thin film of which the surface is terminated with hydrogen is formed on a substrate made of Mo, Pd, Ti or AlN or the like, and the emission efficiency of the second electron is improved.